CAROTENOIDS .4. ABSORPTION, METABOLISM, AND TRANSPORT OF CAROTENOIDS

Carotenoids are currently under intense scrutiny regarding their poten tial to modulate chronic disease risk and prevent vitamin A deficiency , and renewed emphasis has been placed on achieving a better understan ding of the metabolic fate of these compounds in humans, The developme nt of new animal models, and use of human metabolic studies and stable tracer methods have greatly improved our knowledge of how carotenoids are absorbed, metabolized, and transported to tissues; however, many important issues remain unresolved, For example, intestinal uptake of carotenoids occurs by passive diffusion, but the lumenal or intracellu lar factors limiting this process are obscure, The intestinal mucosa p lays a key role in the metabolism of provitamin A carotenoids such as beta-carotene, thus greatly influencing their bioavailability, Most re cent evidence supports a central oxidation mechanism of cleavage of be ta-carotene to retinal in the intestinal mucosa, but the extent and si te(s) of postabsorptive metabolism in the human is unknown, While the human and other species clearly absorb non-provitamin A carotenoids, l ittle is known of the extent and pathways of their metabolism and elim ination, The metabolic fate of cis isomers of beta-carotene is a subje ct of recent interest, since 9-cis retinoic acid can apparently be for med from 9-cis beta-carotene hl vitro and in vivo, Substantial cis-tra ns isomerization of at least small oral doses of 9-cis beta-carotene o ccurs in the human, although the site of isomerization is not yet know n, Carotenoids are transported in plasma exclusively by lipoproteins, with the distribution among lipoprotein classes determined in large pa rt by the physical properties of the carotenoid, The consequences of d ifferential distribution in terms of tissue uptake and retention are n ot clear at present, Improved knowledge of the metabolic fate of carot enoids will assist in the development and testing of hypotheses regard ing their potential to influence biological processes in the human.